Delaying the onset of muscle fatigue associated with functional electrical stimulation

ABSTRACT

The present invention relates a system that can configure a stimulus for functional electrical stimulation (“FES”) to maintain a constant muscle force while delaying the onset of muscle fatigue and a related method of use. The stimulus can be delivered to a nerve via a set of multiple electrode contacts according to a stimulation parameter that maximizes a joint moment associated with the stimulus and minimizes the overlap between pairs of contacts. The joint moment can be related to the muscle force, and the overlap can be related to the onset of muscle fatigue.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.13/918,440, filed Jun. 14, 2013, and entitled SYSTEM AND METHOD FORSTIMULATING MOTOR UNITS, which claims the benefit of U.S. ProvisionalApplication No. 61/659,508, filed Jun. 14, 2012. The entirety of thesubject matter of these applications is hereby incorporated by referencefor all purposes.

TECHNICAL FIELD

The present invention relates generally to functional electricalstimulation (“FES”) and, more specifically, to a FES system that canmaintain a constant muscle force while delaying the onset of musclefatigue and related methods of use.

BACKGROUND

Functional electrical stimulation (“FES”) is a technique that restoresfunction to one or more paralyzed muscles, enabling patients sufferingfrom paralysis due to spinal cord injury (“SCI”), head injury, stroke,or other neurological disorder to participate in activities that wouldnot otherwise be possible. For example, a FES standing system stimulatesmotor units associated with standing to allow a SCI patient to stand fora period of time. For the patient's safety while standing, the FESstanding system must ensure that the knees remain locked by delivering amaximal level of constant stimulation. However, the maximal level ofstimulation can also cause muscle fatigue that can limit the standingtime provided by the FES standing systems.

SUMMARY

In one aspect, the present invention includes a method for functionalelectrical stimulation (“FES”) that can configure a stimulus to beprovided to a nerve via a set of multiple electrode contacts. The actsof the method can be performed by a system comprising a processor (e.g.,a FES system, a component of the FES system, etc). The stimulus can beconfigured according to a stimulation parameter that can be determinedbased on a first model of the joint moment that is based on a sum ofmoments generated from stimulus pulses from each contact and a secondmodel of the overlap between pairs of electrode contacts.

In another aspect, the present invention includes a system that canconfigure a stimulus to be provided via a set of multiple electrodecontacts to produce a constant muscle force while delaying the onset ofmuscle fatigue. The system includes a memory storing machine readableinstructions and a processor, coupled to the memory, to facilitateexecution of the machine readable instructions to at least: provide afirst model of a joint moment based on a sum of moments generated fromstimulus pulses applied to a nerve by each contact of a set of multipleelectrode contacts; provide a second model of an overlap between pairsof contacts; and optimize a cost function based on the first model andthe second model to provide a stimulation parameter that configures thestimulus.

In a further aspect, the present invention includes a non-transitorycomputer-readable device storing instructions executable by anassociated processor to perform operations that configure a stimulus tobe provided to a nerve via a set of multiple electrode contacts toproduce a constant muscle force while delaying the onset of musclefatigue. The operations include: determining a first model of a jointmoment based on a sum of moments generated from stimulus pulses appliedto the nerve by each contact; determining a second model of overlapbetween pairs of contacts; and determining a stimulation parameter thatmaximizes the joint moment and minimizes the overlap based on anoptimization of a function of the first model and the second model.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the present invention will becomeapparent to those skilled in the art to which the present inventionrelates upon reading the following description with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic diagram of an example of a system that canconfigure a stimulus for functional electrical stimulation (“FES”) inaccordance with an aspect of the present invention;

FIGS. 2 and 3 are schematic diagrams depicting examples of electrodesthat each include a set of multiple electrode contacts in accordancewith an aspect of the present invention;

FIG. 4 is a schematic graph depiction of the time-varying joint momentsassociated with an example stimulation paradigm in accordance with anaspect of the present invention;

FIG. 5 is a schematic diagram of an example stimulation generator thatcan configure an electrical stimulus to maintain a constant muscle forcewhile delaying the onset of muscle fatigue in accordance with an aspectof the present invention; and

FIG. 6 is a schematic process flow diagram depicting an example of amethod that can configure an electrical stimulus for FES in accordancewith an aspect of the present invention.

DETAILED DESCRIPTION

The present invention generally relates to functional electricalstimulation (“FES”). The term “FES,” as used herein, generally refers toa technique that can restore a motor function to a paralyzed patient. Asused herein, the term “patient” can refer to any warm-blooded organismincluding, but not limited to, a human being, a pig, a rat, a mouse, adog, a cat, a goat, a sheep, a horse, a monkey, an ape, a rabbit, a cow,etc. More specifically, FES can restore the function to the paralyzedpatient via an application of an electrical stimulus (e.g., one or moretime-varying current waveforms) to multiple motor units that cancoordinate a muscle contraction. The term “motor unit,” as used herein,generally refers to a motor neuron and a skeletal muscle fiberinnervated by the motor neuron, and a “muscle” generally includes aplurality of muscle fibers that are part of a corresponding plurality ofmotor units. As used herein, the term “joint moment” generally refers toa moment of force produced across a joint associated with the musclebased on a force associated with the muscle contraction. The jointmoment is associated with the muscle contraction and associated muscleforce.

Many FES systems apply a constant electrical stimulus (e.g., currentstimulus) to motor units to ensure that a strong muscle contraction ismaintained. However, the utility of these FES systems can be limitedbecause the constant electrical stimulus can induce rapid musclefatigue. By employing a time-varying electrical stimulus (with aplurality of corresponding stimulus waveforms) through an electrode thatincludes a set of multiple electrode contacts (e.g., provided by amulti-contact electrode and/or by a plurality of single contactelectrodes) and producing a stimulation parameter of the electricalstimulus (e.g., a phase shift value, a pulse width value, a pulseamplitude value, a pulse frequency value, a total charge value, and/or adifferent adjustable parameter varying in a sinusoidal manner thatfacilitates selective stimulation of the nerve), the FES system of thepresent invention can stimulate a plurality of mutually agonist motorunits to maintain a strong muscle contraction with constant muscle force(and corresponding constant joint moment) while delaying the onset ofthe muscle fatigue (e.g., by reducing the duty cycle of stimulationand/or minimizing the overlap between individual electrode contacts).

The present invention includes reference to block diagrams and/orflowchart illustrations of methods, apparatus, systems and/or computerprogram products according to certain aspects of the present invention.It is understood that each block of the block diagrams and/or flowchartillustrations, and combinations of blocks in the block diagrams and/orflowchart illustrations, can represent aspects of the present inventionthat can be embodied in hardware and/or in software (including firmware,resident software, micro-code, etc.). Software aspects of the presentinvention can be implemented by computer program instructions that canbe stored in a non-transitory memory (e.g., any non-transitory mediumthat can contain or store the program instructions, including, but notlimited to, a portable computer diskette; a random access memory; aread-only memory; an erasable programmable read-only memory (or Flashmemory); and a portable compact disc read-only memory) and may executedby a processor of a general purpose computer, special purpose computer,and/or other programmable data processing apparatus. Upon execution, thefunctions/acts specified in the block diagrams and/or flowchart block orblocks can be implemented.

In the context of the present invention, the singular forms “a,” “an”and “the” can include the plural forms as well, unless the contextclearly indicates otherwise. It will be further understood that theterms “comprises” and/or “comprising,” as used herein, can specify thepresence of stated features, steps, operations, elements, and/orcomponents, but do not preclude the presence or addition of one or moreother features, steps, operations, elements, components, and/or groupsthereof. As used herein, the term “and/or” can include any and allcombinations of one or more of the associated listed items.Additionally, it will be understood that, although the terms “first,”“second,” etc. may be used herein to describe various elements, theseelements should not be limited by these terms. These terms are only usedto distinguish one element from another. Thus, a “first” elementdiscussed below could also be termed a “second” element withoutdeparting from the teachings of the present invention. The sequence ofoperations (or acts/steps) is not limited to the order presented in theclaims or figures unless specifically indicated otherwise.

In accordance with the present invention, FIG. 1 depicts an example of aFES system 1 that can configure an electrical stimulus of a nerve suchthat one or more components of the nerve can signal corresponding musclefibers to provide a constant muscle force while delaying the onset ofmuscle fatigue. Examples of suitable applications of the FES system 1include, but are not limited to, motor neural prostheses for: standing,walking, hand grasp, upper extremity function, and/or bowl, bladder, anderection control. The FES system 1 can include an electrode 6, astimulator 8, and a stimulation generator 10. The electrode 6, thestimulator 8, and/or the stimulation generator 10 can be configured tocommunicate via one or more wired and/or wireless connections and can beconfigured to be implantable within the patient and/or external to thepatient.

The electrode 6 can be configured to form an electrical interface with acomponent of the peripheral nervous system (the “component”) to deliveran electrical stimulus to a plurality of independent, mutually agonistmotor units associated with a muscle (or “plurality of motor units”) tocontrol the muscle and/or a function of the muscle associated with anapplication of the FES system 1. Examples of the component include, butare not limited to: a peripheral nerve, a fascicle, a synapse, a motorunit, etc. Although an electrode that is configured to be internal tothe patient's body (e.g., an electrode that is in contact with a nerveand/or a muscle) is described herein, it will be understood that theelectrode 6 can be located external to the patient's body (e.g., anelectrode that contacts the skin).

The electrode 6 can be configured to deliver an electrical stimulus tothe component via a set of multiple electrode contacts (also referred toas the “set of contacts”). The set of contacts can include N electrodecontacts, where N is a positive integer greater than or equal to two. Inan embodiment, N can be a positive integer greater than or equal tothree. Each contact can be configured to provide the electrical stimulus(or a portion of the electrical stimulus) to the component to activate acorresponding portion of the plurality of motor units. It will beunderstood that due to the placement and/or shape of electrode 6 and/orthe anatomical shape of the component, one or more of the individualelectrode contacts may not be associated with any portion of theplurality of motor units.

The set of contacts can be provided by a multi-contact electrode and/orby a plurality of single contact electrodes. Accordingly, the set ofcontacts can include, but is not limited to, a multi-contact nerve cuffelectrode (examples schematically illustrated in FIGS. 2 and 3), one ormore epimysial electrodes, one or more intramuscular electrodes, one ormore electrode arrays, and/or one or more intrafasicular electrodes.However, any type of electrode that can achieve a stable, selective,chronic interface with the associated component of the peripheralnervous system to enable control of a muscle and/or a functionality ofthe muscle is within the scope of the present invention.

One example type of nerve cuff electrode, as illustrated in FIG. 2, is aself-sizing spiral nerve cuff electrode 30 (the “cuff electrode”) thatis configured to wrap around a portion a nerve, as shown in FIG. 2. Thecuff electrode 30 has a circular cross-section and four electrodecontacts 6 a-6 d that are spaced equally around the circumference of thenerve. Because the cuff electrode 30 is circular, it is well suited forinterfacing with small circular nerves. However, the cuff electrode 30may not be as appropriate for larger and/or non-circular (e.g., flatter)nerves at least because the electrode contacts 6 a-6 d preferentiallystimulate a portion of the fascicles 34 a-g within the nerve, but canleave other fascicles 36 a-d (within the center of the cross section)unstimulated and/or stimulated to a lesser extent.

Another example type of nerve cuff electrode, as illustrated in FIG. 3,is a flat interface nerve electrode (the “FINE”) 40. In the exampledepicted in FIG. 3, the FINE 40 has a rectangular cross-section, whichaligns the fascicles 42 a-42 i so that they are close enough to theeight stimulating contacts 42 a-42 h so that all of the fascicles can bestimulated. Accordingly, the FINE 30 may be better suited for larger andflatter nerves than the cuff electrode 30.

Referring again to FIG. 1, the stimulator 8 can be configured toconfigure the electrical stimulus according to a stimulation parameter28 and to provide the configured electrical stimulus to the electrode 6(e.g., via a stimulator assembly) for stimulation of the plurality ofmotor units. The stimulation generator 10 can be configured to adjustthe stimulation parameter 28 to vary the number of motor units recruitedat a time according to the electrical stimulus by each contact. Thestimulation generator 10 can be located within the stimulator 8 and/orcan be located independent from the stimulator 8. The stimulator 8and/or the stimulation generator 19 can be configured to provide acontrol system and/or a power system for FES system 1. Although thestimulator 8 and the stimulation generator 10 are described as externalto the patient's body herein, it will be understood that one or both ofthe stimulator 8 and/or the stimulation generator 10 can be implantablewithin the patient's body.

The electrical stimulus can include one or more time-varying currentwaveforms that can stimulate the plurality of motor units to induce oneor more time-varying joint moments that can result in a constant jointmoment that can correspond to a constant muscle force. In an embodiment,the electrical stimulus can include a plurality of time-varying currentwaveforms (the “plurality of waveforms”). As used for exemplary purposesherein, the plurality of waveforms can include sinusoidal waveforms.However, it will be understood that the plurality of waveforms are notlimited to sinusoidal waveforms and can include other types of timevarying current waveforms associated with one or more parameters thatcan be adjusted to produce a constant muscle force while delaying theonset of muscle fatigue (e.g., carousel waveforms, interleavedwaveforms, etc.).

The stimulation parameter 28 can be associated with the one or moreparameters related to the plurality of waveforms that can be deliveredby the contacts to achieve selective stimulation of the plurality ofmotor units. In an example where the waveforms are sinusoidal waveforms,the stimulation parameter 28 can include, but is not limited to: a phaseshift value, a pulse width value, a pulse amplitude value, a pulsefrequency value, a total charge value, etc.

One example of an electrical stimulation that can be generated andconfigured by the stimulator 8 according to the stimulation parameter 28is a sum of phase shifted sinusoids (the “SOPS”) electrical stimulation.The stimulation parameter 28 of the SOPS electrical stimulationwaveforms can be a pulse width parameter. The waveforms with differentpulse widths can cause the joint moments generated by multipleindependent motor unit populations to oscillate with equal amplitude andfrequency, but offset phases, so that their combined output is aconstant value equal to the sum of their average joint moments. Forelectrodes that can stimulate two or more independent populations ofmotor units, the total joint moment produced with the SOPS electricalstimulation will, by definition, be greater than the contribution of anysingle independent population of motor units. This allows a reduction induty cycle as compared to constant stimulation and a correspondingreduction in fatigue, while still providing a high total joint moment.The SOPS electrical stimulation exhibits an improved circulation andoxygenation of the muscle tissue, a reduction of acidosis, a delay indepletion of glycogen and ATP stores within the muscle, and subsequentdelay in the onset of muscle fatigue.

The SOPS electrical stimulation employs the mathematical identity thatthe sum of multiple sinusoids with equal amplitude and frequency withphase shift values evenly spread between 0° and 360° (0 and 2pi radians)will be constant and cancel the oscillations (or “ripple”) in theresultant joint moment. As shown in FIG. 4, the moments generated bystimulating each of the three independent populations of motor units(e.g., 52, 54 and 56) oscillate between on and off and, because of theirevenly distributed phase shifts values, the oscillations cancel out,resulting in a ripple-free constant joint moment 58 equal to the sum ofthe offsets of the oscillations.

Below is the mathematical derivation of an example of this relationshipfor three independent motor units or populations of motor units, but asimilar approach could be taken for any number of independentpopulations of motor units greater than or equal to two.

Three isometric joint moments (M_(A), M_(B) and M_(C)) with oscillatingmagnitude, constant moment arm, and equally distributed phase aredefined as:M _(A) =r _(A)(α_(A) sin(πt)+α_(A));  (Eq. 1)M _(B) =r _(B)(α_(B) sin(πt+⅔π)+α_(B));  (Eq. 2)M _(C) =r _(C)(α_(C) sin(πt+4/3π)+α_(C));  (Eq. 3)where M_(n) is the joint moment produced by contact n, r_(n) is themoment arm for motor units activated by contact n, α_(n) is theamplitude of the peak force generated by contact n, and α_(n) is theoffset of the oscillations of contact n.

If all moments are agonists, and all moment arms are equal:

$\begin{matrix}\begin{matrix}{M_{total} = {M_{A} + M_{B} + M_{C}}} \\{= {{r\begin{pmatrix}{{a_{A}{\sin\left( {\pi\; t} \right)}} + \alpha_{A} + {\alpha_{B}\sin\left( {{\pi\; t} + {{2/3}\pi}} \right)} +} \\{\alpha_{B} + {a_{C}{\sin\left( {{\pi\; t} + {{4/3}\pi}} \right)}} + \alpha_{C}}\end{pmatrix}}.}}\end{matrix} & \left( {{Eq}.\mspace{14mu} 4} \right)\end{matrix}$

Using the trigonometric identity:

$\begin{matrix}{\mspace{79mu}{{{\sin\left( {m + n} \right)} = {{{\sin(m)}{\cos(n)}} + {{\cos(m)}{\sin(n)}}}},}} & \left( {{Eq}.\mspace{14mu} 5} \right) \\{{M_{total} = {r\left( {{a_{A}{\sin\left( {\pi\; t} \right)}} + \alpha_{A} + {a_{B}\left( {{{\sin\left( {\pi\; t} \right)}{\cos\left( {{2/3}\pi} \right)}} + {{\cos\left( {\pi\; t} \right)}{\sin\left( {{2/3}\pi} \right)}}} \right)} + \alpha_{B} + {a_{C}\left( {{{\sin\left( {\pi\; t} \right)}{\cos\left( {{4/3}\pi} \right)}} + {{\cos\left( {\pi\; t} \right)}{\sin\left( {{4/3}\pi} \right)}}} \right)} + \alpha_{C}} \right)}},} & \left( {{Eq}.\mspace{14mu} 6} \right)\end{matrix}$which can be simplified to:

$\begin{matrix}{M_{total} = {{r\left( {{a_{A}{\sin\left( {\pi\; t} \right)}} + \alpha_{a} + {a_{B}\left( {{{{- 1}/2}{\sin\left( {\pi\; t} \right)}} + {{\sqrt{3}/2}{\cos\left( {\pi\; t} \right)}}} \right)} + \alpha_{B} - {a_{C}\left( {{{1/2}{\sin\left( {\pi\; t} \right)}} + {{\sqrt{3}/2}{\cos\left( {\pi\; t} \right)}}} \right)} + \alpha_{C}} \right)}.}} & \left( {{Eq}.\mspace{14mu} 7} \right)\end{matrix}$

If α_(A)=α_(B)=α_(C), then the sin(πt) terms and the cos(πt) termscancel out, leaving:M _(total) =r(α_(A)+α_(B)+α_(C))  (Eq. 8)

In an implementation of the SOPS electrical stimulation with threeindependently stimulated motor units, each of the first, second, andthird sinusoidally-varying moments can have a same amplitude, such thata sum of the moments provided across all of the plurality of motor unitsis substantially constant and non-zero. In accordance with an aspect ofthe invention, appropriate first, second, and third stimulation currentscan be determined according to a model representing recruitment andoverlap characteristics of first, second, and third motor units andtheir associated electrodes.

In order to achieve sinusoidal oscillations in joint moment, for eachcontact a pulse-width modulated isometric tetanic recruitment curve canbe used as a transfer function between the desired joint moment and anestimate of the stimulation pattern required to generate it. Forexample, the recruitment curve can be created by applying three secondtrains of stimulus pulses through each contact with varied pulse widths(e.g., ranging from 1 to 255 μs) while the joint is held at a constantflexion and the joint moment is measured with respect to each stimuluspulse. The SOPS electrical stimulation can achieve ripple-free jointmoment if the maximum joint moments generated by stimulating through allcontacts are equal.

For example, if there are three independent agonist populations of motorunits corresponding to three electrode contacts, and the joint momentproduced by stimulating each population is made to oscillate at 0.2 Hzbetween 0 and 5 Nm, as long as the three sinusoids have phase shifts of0°, 120° or 240°, the summed output will be ripple-free, with only thesum of their offsets remaining. This example would produce a constant7.5 nm of knee extension moment, since the offset for each of the threesinusoids is 2.5 Nm.

Referring again to FIG. 1, the stimulation generator 10 can receivemoment data 16 corresponding to the joint moment associated with theelectrical stimulation and overlap data 18 associated with theelectrical stimulation, and can adjust the stimulation parameter 28based on the moment data 16 and the overlap data 18. An exampleconfiguration of the stimulation generator 10 is illustrated in FIG. 5.The stimulation generator 10 can include a memory 14 (e.g., anon-transitory computer readable medium) to store computer programinstructions that correspond to the adjustment of the stimulationparameter 28 and a processor 16 to execute the computer programinstructions to adjust the stimulation parameter 28.

To achieve an electrical stimulation that can provide a constant muscleforce (corresponding to a constant joint moment—by minimizing a rippleassociated with a sinusoidal stimulation waveform) while delaying theonset of muscle fatigue (e.g., by reducing the duty cycle of stimulationand/or reducing the overlap between electrode contacts), the stimulationgenerator 10 can adjust the stimulation parameter 28 to maximize thejoint moment and minimize the overlap. Accordingly, the stimulationgenerator 10 can determine a first model of the joint moment (“jointmoment model” 20) based on the moment data 16 and a second model of theoverlap (“overlap model” 22) based on the overlap data 18.

The moment data 16 can include data related to moments generated fromindividual contacts of the set of contacts. For example, the individualcontacts of the set of contacts can receive a stimulus pulse andcorresponding resultant joint moment can be recorded. The moment data 16can also include a sum of the individual moments generated based on thestimulus pulses applied to each of the contacts individually. Theoverlap can correspond to pairs of contacts recruiting the same motorunits, thereby increasing the likelihood of fatigue in the motor unitsthat are jointly recruited.

The overlap data 18 can include data corresponding to an overlap betweenpairs of contacts from the set of contacts (e.g., shown betweenstimulations of the individual contacts delayed by a short timeinterval). The overlap data 18 can exclude data related to adjacentcontacts from the set of contacts. For example, pairs of contacts of theset of contacts that are not adjacent to each other can be determinedfrom the overlap model 22.

The stimulation generator 10 can employ a cost function 24 based on thejoint moment model 20 and the overlap model 22 to adjust the stimulationparameter 28 to a value that creates the electrical stimulus that canproduce a maximal joint moment and a minimal overlap. In an example, thecost function 24 can be a function of the stimulation parameter 28. Thecost function can also be a function of the joint moment model 20 andthe overlap model 22 (which can be scaled by weighting factors). Thecost function can be, but is not limited to, a polynomial function, asigmoid function, a Gaussian function, a Gompertz function, and/or amulti-dimensional polynomial function

An example derivation of a cost function by the stimulation generator 10based on the joint moment model 20 and the overlap model 22 is describedbelow. In this example, the stimulation parameter 28 can be the pulsewidth of the sinusoidal stimulation waveforms (however, otherstimulation parameters can form cost functions in a similar manner).

The cost function 24 can be of the form:C( PW )=−ω₀ M _(T)( PW )+ω₁ O _(T)( PW ),  (Eq. 9)where PW is an N-dimensional vector of pulse widths of stimulus pulsesfor an N-contact electrode, O_(T) quantifies the overlap of all contactswithin the electrode, M_(T) quantifies the joint moment generated by allcontacts within the electrode, and ω_(o) and ω₁ are weighting factorsthat can be chosen to emphasize larger joint moments or smaller overlap.A penalty term can be added to the cost function in cases where thejoint moment is less than a first threshold value (e.g., 5 Nm) and/orthe overlap is greater than a second threshold value (e.g., 10%).

The joint moment term (or joint moment model 20), M_(T), can be definedin this example as:

$\begin{matrix}{{{M_{T}\left( \overset{\_}{PW} \right)} = \frac{\sum\limits_{i = {1:N}}{M_{i}\left( {PW}_{i} \right)}}{\sum\limits_{i = {1:N}}{\max\left( M_{i} \right)}}},} & \left( {{Eq}.\mspace{14mu} 10} \right)\end{matrix}$where M_(i) is the moment generated when stimulating through contact i.As an example, M_(i) can be based on data related to recruitment ofmotor units by the corresponding stimulus. The data related to therecruitment can be based on twitch responses of the muscle associatedwith the joint moment associated. The twitch response can be linearlyrelated to a tetanic response (corresponding to the constant muscleforce and constant joint moment); for example, the twitch response andthe tetanic response can be related according to a linear scaling factorthat can be determined based on a ratio of a maxima of a twitch responsecurve to a maxima of a tetanic response curve. The data related to therecruitment can, additionally or alternatively, be defined by a user.

In the M_(T) function, the sum of the moment functions from stimulatingthrough each contact is divided by the sum of the maxima of the momentfunctions to normalize the joint moment term. In this way, the overalljoint moment is normalized with respect to the overlap term, but jointmoments from each of the contacts are treated equally with respect toeach other.

The overlap term (or overlap model 22) can be defined in this example asfollows. Overlap for a pair of contacts is quantified by the deviationfrom linear addition when stimulation is applied through one contactshortly after stimulation through another contact. This can be expressedas:M _(i∩j)(PW _(i) ,PW _(j))=M _(i)(PW _(i))+M _(j)(PW _(j))−M _(i∪j)(PW_(i) ,PW _(j))  (Eq. 11)where M_(i∩j) is the overlap between contacts i and j, M_(i) and M_(j)are the moments generated when stimulating through contacts i and j,respectively, and M_(i∪j) is a mathematical function fit to the momentgenerated when stimulating through two contacts with a short time delay.To take all of these pairwise overlaps into account, while normalizingthe overlap so that its weighting is controlled relative to M_(T), O_(T)is defined as:

$\begin{matrix}{{O_{T}\left( \overset{\_}{PW} \right)} = {\frac{2}{N^{2} - N}{\sum\limits_{i = {1:{N - 1}}}{\sum\limits_{J = {2:N}}\frac{M_{i\bigcap j}\left( {{PW}_{i},{PW}_{j}} \right)}{M_{i\bigcup j}\left( {{PW}_{i},{PW}_{j}} \right)}}}}} & \left( {{Eq}.\mspace{14mu} 12} \right)\end{matrix}$which ranges between 0 and 1. The overlap between pairs of contacts canvary over time (e.g., as a result of changes in the electrode-nerveinterface, because the relative strength of the response throughindividual contacts is used in the calculation of the overlap, etc.), sothe overlap model may be recalculated periodically or at any time deemednecessary over an extended period of time.

The stimulation generator 10 can also include an optimizer 26 that canadjust the stimulation parameter 28 in view of an optimization of thecost function 24. For example, the optimizer 26 can minimize the costfunction 24 to determine the stimulation parameter 28 that canconfigured the electrical stimulus to produce a constant contraction (orconstant joint moment) with a delayed onset of fatigue. In the casewhere the stimulation waveforms are sinusoidal, the stimulationparameter can be chosen such that the constant contraction can be acontraction that is virtually free from ripple.

The optimizer 26 can employ an optimization algorithm to optimize thecost function 24 so that the stimulation parameter 28 can be determined.The optimization algorithm can minimize the cost function 24 todetermine an optimal stimulation parameter 28 that can maximize thejoint moment and minimize the overlap. As an example, the optimizationalgorithm can be a direct search optimization algorithm. It will beunderstood that other types of optimization algorithms can be used toadjust the stimulation parameter.

While aspects of the present invention have been particularly shown anddescribed with reference to the embodiment of system 1, it will beunderstood by those of ordinary skill in the art that various additionalembodiments may be contemplated without departing from the spirit andscope of the present invention. For example, the specific configurationand application of system 1 are merely illustrative; one of ordinaryskill in the art could readily determine any number of tools, sequencesof steps, or other means/options for placing the above-describedapparatus, or components thereof, into positions substantively similarto those shown and described herein. Any of the described structures andcomponents could be integrally formed as a single unitary or monolithicpiece or made up of separate sub-components, with either of theseformations involving any suitable stock or bespoke components and/or anysuitable material or combinations of materials such as, but not limitedto, stainless steel, titanium, platinum, Nitinol, epoxies, urethanes,metals, polymers, ceramics, and the like; however, the chosenmaterial(s) should be biocompatible for many applications of the presentinvention. Nerves, muscles, fascicles, and/or any other stimulatedstructures of the living body are described herein without restrictionas “motor units” and/or “nerves”, due to the integrated and connectednature of all of these structures with respect to the described useenvironments. Though certain components described herein are shown ashaving specific geometric shapes, all structures of the presentinvention may have any suitable shapes, sizes, configurations, relativerelationships, cross-sectional areas, or any other physicalcharacteristics as desirable for a particular application of the presentinvention. Any structures or features described with reference to oneembodiment or configuration of the present invention could be provided,singly or in combination with other structures or features, to any otherembodiment or configuration, as it would be impractical to describe eachof the embodiments and configurations discussed herein as having all ofthe options discussed with respect to all of the other embodiments andconfigurations. Other electrode designs and stimulation paradigms couldbe provided, such as, but not limited to, field steering, bipolar ortripolar electrode configurations, and/or different geometries such as aflat cuff cross-section to further improve selectivity and performance.

In view of the foregoing structural and functional features describedabove, a method 60 in accordance with various aspects of the presentinvention will be better appreciated with reference to FIG. 6. While,for purposes of simplicity of explanation, the method 60 of FIG. 6 isshown and described as executing serially, it is to be understood andappreciated that the present invention is not limited by the illustratedorder, as some aspects could occur in different orders and/orconcurrently with other aspects shown and described herein. Moreover,not all illustrated aspects may be required to implement method 60. Itwill be appreciated that the illustrated aspects of method 60 can beimplemented in whole or in part as machine-executable instructionsstored on a non-transitory computer readable device (e.g., memory 14).The instructions can be executed by a processing device (e.g., processor12) to facilitate the performance of the operations of the method 60.

FIG. 6 illustrates an example of a method 60 for configuring a stimulusto be provided to a nerve via a set of multiple electrode contacts(e.g., provided by electrode 6, which can be a multi-contact electrodewith contacts 6 a-h or 6 i-p and/or a plurality of single-contactelectrodes) for functional electrical stimulation (e.g., to produce aconstant muscle force while delaying the onset of muscle fatigue). At62, a first model of a joint moment (e.g., joint moment model 20) isdetermined (e.g., by stimulation generator 10) based on a sum of moments(e.g., moment data 16) generated from stimulus pulses applied to a nerveinnervating a muscle by each contact (e.g., each contact receives therespective stimulus pulse in sequence) of a set of contacts. At 64, asecond model of an overlap between pairs of contacts (e.g., adjacentpairs of contacts) of the set of contacts (e.g., overlap model 22) isdetermined (e.g., by stimulation generator 10) based on datarepresenting the overlap (e.g., overlap data 18). At 66, based on thefirst model and the second model, a stimulation parameter (e.g.,stimulation parameter 28) is determined (e.g., by optimizer 26 ofstimulation generator 10) that maximizes the joint moment and minimizesthe overlap.

From the above description, those skilled in the art will perceiveimprovements, changes and modifications. Such improvements, changes andmodifications are within the skill of one in the art and are intended tobe covered by the appended claims.

Having described the invention, we claim:
 1. A method, comprising:determining, by a system comprising a processor, a stimulation parameterfor a set of electrical stimulus waveforms to be applied to a nerve by aset of multiple electrode contacts, comprising: determining a firstmodel of a joint moment based on a sum of moments generated from anelectrical stimulus pulse from each contact of a set of multipleelectrode contacts; determining a second model of an overlap betweenpairs of contacts of the set of multiple electrode contacts; anddetermining the stimulation parameter based on a function of the firstmodel and the second model, wherein the stimulation parameter maximizesthe joint moment and minimizes the overlap; and configuring, by thesystem, the set of electrical stimulus waveforms to be applied to thenerve by the set of multiple electrode contacts based on the stimulationparameter, wherein a stimulation of the nerve by the set of multipleelectrode contacts applying the set of electrical stimulus waveformsresults in a constant joint moment.
 2. The method of claim 1, whereinthe sum of moments corresponds to a sum of twitch responses of themuscle to the stimulus pulses from each contact of a set of multipleelectrode contacts.
 3. The method of claim 1, wherein the stimulationparameter comprises at least one of a phase shift value, a pulse widthvalue, a pulse amplitude value, a pulse frequency value, a total chargevalue, or an adjustable parameter varying in a sinusoidal manner thatfacilitates selective stimulation of the nerve.
 4. The method of claim1, wherein the set of electrical stimulus waveforms comprises aplurality of sinusoidal waveforms each comprising a phase that is evenlyspread between 0 degrees and 360 degrees.
 5. The method of claim 1,wherein the stimulation parameter delays an onset of fatigue of a muscleassociated with the joint.
 6. The method of claim 1, wherein thefunction comprises a polynomial function, a sigmoid function, a Gaussianfunction, a gompertz function, a multi-dimensional polynomial function,or another appropriate mathematical representation.
 7. The method ofclaim 1, wherein the stimulation parameter minimizes a ripple in thejoint moment over time.
 8. The method of claim 1, wherein thedetermining the second model further comprises eliminating non-adjacentpairs of contacts of the set of multiple electrode.
 9. A system,comprising: a memory storing machine readable instructions; and aprocessor, coupled to the memory, to facilitate execution of the machinereadable instructions to at least: provide a first model of a jointmoment based on a sum of moments generated from stimulus pulses appliedto a nerve innervating a muscle by each contact of a set of multipleelectrode contacts; provide a second model of an overlap between pairsof contacts of the set of multiple electrode contacts; optimize a costfunction based on the first model and the second model to provide astimulation parameter that maximizes the joint moment and minimizes theoverlap; and configure a set of electrical stimulus waveforms to beapplied to a nerve by the set of multiple electrode contacts based onthe stimulus parameter, wherein a stimulation of the nerve by the set ofmultiple electrode contacts applying the set of electrical stimuluswaveforms results in a constant joint moment.
 10. The system of claim 9,wherein the sum of moments corresponds to a sum of twitch responses ofthe muscle to the stimulus pulses.
 11. The system of claim 9, whereinthe overlap is identified for each pair when the corresponding pair ofmoments deviates from linear addition.
 12. The system of claim 9,wherein the set of multiple electrode contacts comprises at least 2electrode contacts.
 13. The system of claim 9, wherein the stimulationparameter delays an onset of fatigue of a muscle associated with thejoint by intermittently activating the at least two independent motorunits.
 14. The system of claim 9, wherein the model of joint moment orthe model of overlap is user definable.
 15. A non-transitorycomputer-readable device storing instructions executable by anassociated processor to perform operations, comprising: determining afirst model of a joint moment based on a sum of moments generated fromstimulus pulses applied by each contact of a set of multiple electrodecontacts; determining a second model of overlap between pairs ofcontacts of the set of multiple electrode contacts; and determining astimulation parameter that maximizes the joint moment and minimizes theoverlap based on an optimization of a function of the first model andthe second model; and configuring a set of electrical stimulus waveformsto be applied to a nerve by the set of multiple electrode contacts basedon the stimulation parameter to ensure a constant joint moment.
 16. Thenon-transitory computer-readable device of claim 15, wherein the overlapis related to a fatigue of a muscle associated with the nerve.
 17. Thenon-transitory computer-readable device of claim 16, wherein the overlapis related to the pairs of contacts recruiting the same motor unit. 18.The non-transitory computer-readable device of claim 15, wherein thefunction is a cost function of the first model and the second model, andthe providing the stimulation parameter further comprises minimizing thecost function to provide the stimulation parameter.
 19. Thenon-transitory computer-readable device of claim 16, wherein thestimulation parameter comprises an adjustable parameter of a pluralityof sinusoidal stimulation waveforms in the sum of phase shiftedsinusoids electrical stimulation.
 20. The non-transitorycomputer-readable device of claim 19, wherein the common featurecomprises a common amplitude or a common frequency.